1. Field of the Invention
This invention relates to an oxygen sensor used for example, for emission control of an automoble engine.
2. Description of the Prior Art
A usual oxygen sensor of the solid electrolyte type which is used for sensing oxygen gas concentration in an exhaust gas is shown in FIG. 1. The sensor has an oxygen ion conductive solid electrolyte element body 1 (sensing body), formed for example of ceramic material, in the shape of a tube sealed at one end. The element body 1 has a pair of platinum electrodes 3 and 2 respectively on the exhaust gas side (the outer side) and the reference gas side (the inner side or air side).
The sensing element formed of the body 1 and the electrodes 2 and 3 is fitted in an exhaust manifold of an automoble engine through an electrically conductive holder 4 and a flange 5 so as to be set in the position where the sealed end of it can be exposed to exhaust gas flow.
The inner electrode 2 of the element is in electrical contact with a terminal 7 of a good electric conductor which is shaped like a pipe through a graphite ring 6. The air can come into the inner electrode 2 (reference which is on the gas side of the element) through the pipe of the terminal 7.
The outer electrode 3 is in electrical contact with the holder 4 of a good electric conductor through a graphite ring 8. In order to prevent leakage of the exhaust gas, a packing ring 9 in addition to the graphite ring 8 is set between the element and the holder 4.
By using the above oxygen sensor, an oxygen concentration can be sensed in the exhaust gas from the e.m.f. value which is generated between the electrodes 2 and 3 due to a difference between the oxygen concentrations of the exhaust gas and the air. The e.m.f. value E is expressed by the following equation: EQU E=(RT/4F) log (P.sub.1 /P.sub.2)
where P.sub.1 and P.sub.2 designate oxygen partial pressures of the air and the measured exhaust gas, respectively. T, R, and F denote an absolute temperature, gas constant, and Faraday's constant, respectively.
The outer electrode 3 is usually exposed to exhaust gas of high temperature and high pressure.
In general, in a usual oxygen sensor, a ceramic coating 10 having porous structure is formed as a cover on the surface of the outer electrode 3 by means of a plasma spray method in order to protect the electrode against damage of exhaust gas of high temperature and to slow down the rate of the gas flow toward the electrode. How much the gas flow rate should be slowed down depends on the particular engine. In forming the ceramic coating 10, it is important to make uniform the thickness thereof, because otherwise the degree of gas flow rate control varies from position to position on the ceramic coating.
The slowing down of the gas flow rate is necessary for lengthening the response time of the sensing element, which lengthening is necessary for making the sensing element response time match with the mechanical response time of the fuel supply mechanism in the engine. However, it is technically difficult to form a uniformly porous ceramic coating.
Further, conventionally, the sensor has two more protecting covers 11(a) and 11(b) with many holes, namely an outer cover 11(a) and an inner cover 11(b) as shown in FIG. 2, in order to protect the electrode 3 and the sensing body 1 from sudden thermal and mechanical shock of the exhaust gas flow, which are generated by combustion of fuel in the cylinders of the engine.
Recently, demand for sensors with a decreased number of protecting covers has arisen for the purpose of reducing the total price of the sensor. However, it is found that the sensing element in such a sensor, e.g., without an inner cover 11(b), is damaged by thermal shock because when the ceramic coating 10 is directly exposed to the exhaust gas flow, the ceramic coating (usually together with the sensing element) is likely to be broken. Further, when the ceramic coating 10 is removed, the sensing response time cannot be controlled.